If that's what he says it is, make him rebalance the wheels before you buy it. If that's what it turns out to be, just add $20 to the purchase price. If that does not solve the problem, then dig deeper or look for another car.

At 43,000 miles on an ITR that was used only for general transportation, it's very unlikely that the wheel bearings or the axle joints are going. They are fairly heavy duty items and unless the car was constantly driven on road courses, you're probably OK...but make him get the wheels balanced to be sure!